Conventionally, a lactic acid component is separated from a lactic acid fermentation liquor using, for example, a method for collecting a lactic acid component in the form of a lactic acid ester, a method for collecting a lactic acid component in the form of calcium lactate, or a method for separating a lactic acid component by electrodialysis.
As the method for collecting a lactic acid component in the form of a lactic acid ester, a method is disclosed in which a lactic acid ester is synthesized by adding butanol or pentanol to ammonium lactate generated by fermentation, and collected by distillation (for example, Japanese Laid-Open Patent Publication No. 6-311886).
As the method for collecting a lactic acid component in the form of calcium lactate, a method is disclosed in which lactic acid obtained from fermentation is neutralized with calcium hydroxide or calcium carbonate to generate calcium lactate precipitates for collection (for example, Japanese Laid-Open Patent Publication No. 6-070679).
As the method using electrodialysis, a method is disclosed in which a lactic acid fermentation liquor is introduced into a desalting chamber of an electrodialysis cell with which an anode, a cathode, and alternately arranged cation exchange membranes and anion exchange membranes are provided, and thus salts of lactic acid is concentrated in the fermentation liquor and separated (for example, Japanese Laid-Open Patent Publication No. 7-155191).
Japanese Laid-Open Patent Publication Nos. 2002-300898 and 10-287668 disclose methods for producing polylactic acid from fermented lactic acid products as a starting material. Japanese Laid-Open Patent Publication No. 2002-300898 discloses a method for producing a polylactic acid by esterifying ammonium lactate products from lactic acid fermentation with an alcohol and polycondense them, and then producing lactides and polymerizing them. Japanese Laid-Open Patent Publication No. 10-287668 discloses a method for producing lactide by heating ammonium lactate under reduced pressure, and a method for producing a polylactic acid by ammonia evaporation and dehydratively condensation-polymerization. All of these methods are described as polymerization methods without converting fermented lactic acid products (ammonium lactate salt) from a salt form into a free form. Ammonium lactate is azeotropically dehydrated with xylene to produce polylactic acid, and then polylactic acid with a weight-average molecular weight of 70000 is treated with methylene chloride and methanol for crystallization.
In conventional methods for separating a lactic acid component from a lactic acid fermentation liquor, it is required to separate and purify the lactic acid component by causing a reaction by adding a further reaction reagent, for example, an alcohol such as butanol or pentanol, or a calcium salt such as calcium hydroxide or calcium carbonate. Thus, except for the case of obtaining lactic acid esters or calcium lactate as a final target product, the further reagents may be used with increase in the number of steps, the cost of agents, and the possibility of incorporation of concomitant chemical substances or impurities. For example, when a separated lactic acid component is used as a starting material of chemicals such as polylactic acid or polyester polyol, it is required to separate lactic acid esters produced in the reaction by distillation, and then hydrolyze them to lactic acid as the pre-reaction form. The resultant alcohols are required to be removed since alcohols cannot be contained in the final product.
In the separation method using electrodialysis, there are a concern about the life and fouling of the membranes used. Furthermore, a lactic acid fermentation liquor may contain not only lactic acid, but also organic components such as proteins and surfactants and inorganic components such as salts of phosphoric acid, sulfuric acid, acetic acid, and citric acid, as nutrients and medium components for lactic acid bacteria. Thus, contamination of the membranes is severer, and maintenance of the membranes leads to an increase in the cost. In addition, salts derived from medium components in the fermentation liquor, as well as the lactate salts, are concentrated by dialysis, and thus the precision of separation and the purity of the lactate salt are lowered.